1. Field of the Invention
The present invention relates to an optical system having a plurality of optical elements, a method of producing the optical system, and an optical pickup having the optical system.
2. Description of the Related Art
In recent years, there has been a demand for optical storage media having a higher density. Therefore, research and development have been underway in optical disk apparatuses for shortening the wavelength of the light source and increasing the numerical aperture (NA) of the reproduction optical system. Also, there has been a demand for a higher transfer rate of data.
When shortening the wavelength of the light source and raising the numerical aperture (NA) of the reproduction optical system, the size of the optical spot becomes smaller and the depth of focus becomes shallower, so it is desirable to reduce the error in focal servo control. Further, the width of the optical storage medium on which data is stored (track width) becomes narrower, so it is desirable to reduce the error in tracking servo control.
Further, when raising the transfer rate of data, it is desirable to raise the range of the actuator performing the focal servo control and tracking servo control. As a result, two improvements in the servo control characteristics are desirable: to reduce the error and to improve the range. The actuator can be improved in servo control characteristics by reducing the weight of the actuator.
An optical system having a high NA of 0.85 can be formed by a two-group lens comprised of a combination of two optical lenses. Furthermore, a near field optical lens of an NA=1.5 can be formed.
However, reduction of the size of an optical system means that the curvature of the optical lenses becomes smaller, so the optical lenses are required to be highly precise. When constructing a two-group lens, the requirement of the accuracy of positioning the two optical lenses becomes severe.
FIG. 1 is a schematic view of the configuration of an example of an optical system comprised of a two-group lens.
The optical system 10 comprises optical lenses 1 and 6 and a lens holder 2.
The optical lens (front lens) 1 and optical lens 6 formed by glass molding are positioned and fixed in place via the lens holder 2. The lens holder 2 is prepared for example by resin molding or cutting etc.
The outer circumference portion 1C of the front lens is fit in a recess 2H of the lens holder 2. The outer circumference portion 1C of the front lens and the lens holder 2 are bonded together by an adhesive 4.
A circumferential wall 2C of the lens holder 2 and an outer circumference portion 6C of the optical lens 6 are bonded together by an adhesive 5.
The optical lens 6 is formed with a convex portion 6A on one surface and is formed with a convex portion 6B on the other surface. The convex portion 6B faces the convex portion 1A of the front lens 1.
In the optical system 10, the optical axis of the front lens 1 mounted in the lens holder 2 and an upper surface of the circumferential wall 2C of the lens holder 2 are not always vertical. Therefore, when producing (assembling) the optical system 10, it is necessary to adjust for the skew (adjust the optical axis) of the front lens 1 mounted in the lens holder 2 and the optical lens 6. A clearance necessary for the skew adjustment is provided between the lens holder 2 and the optical lens 6. The clearance is adjusted by the thickness of the adhesive 5.
Originally, it is desirable to increase the bonding area and reduce the thickness of an adhesive. However, in the optical system 10, the adhesive 5 is made a thickness more than necessary for bonding, so the amount of the adhesive 5 increases undesirably. Also, the adhesive 4 and 5 is used to bond a plurality of locations, so assembly of the optical system 10 is troublesome.
A front lens comprised of a glass material can be produced using glass molding by charging the glass material in a mold, then heating and/or pressing the mold. For the glass material, it is possible to use a spherical glass material because it is easy to adjust the volume and the material easily fills the mold.
However, with glass molding, it is difficult to make the outer circumference portion 1C large. Also, skew adjustment of the two-group lenses is complicated and troublesome because it is hard to make the outer circumference portion 1C large in the front lens 1 which has a small curvature of the convex portion 1A.
A first object of the present invention is to provide an optical system capable of reducing the trouble of assembly, a method of producing the optical system, and an optical pickup having the optical system.
A second object of the present invention is to provide an optical system capable of reducing the trouble in assembly and skew adjustment.
According to a first aspect of the present invention, there is provided an optical system comprising first and second optical lenses arranged so as to have coinciding or substantially coinciding optical axes, wherein the first optical lens has a substrate comprised of an optical material; the substrate has a convex portion serving as a convex lens and an outer circumference portion positioned around the convex portion; a thickness of the substrate at the outer circumference portion is greater than a thickness of the substrate at the convex portion; and the outer circumference portion of the first optical lens and the second optical lens are fixed in place so that the convex portion of the first optical lens faces the second optical lens.
Preferably, the outer circumference portion of the first optical lens faces the outer circumference portion of the second optical lens; and a facing surface of the outer circumference portion of the first optical lens and a facing surface of the outer circumference portion of the second optical lens are flat or approximately flat.
More preferably, a facing surface of the outer circumference portion of the first optical lens is vertical or substantially vertical with respect to an optical axis of the first optical lens; a facing surface of the outer circumference portion of the second optical lens is vertical or substantially vertical with respect to an optical axis of the second optical lens; and a facing surface of the outer circumference portion of the first optical lens and a facing surface of the outer circumference portion of the second optical lens are bonded together.
Preferably, the outer circumference portion is positioned around the flat portion; and a thickness of the substrate at the outer circumference portion is greater than a thickness of the substrate at the flat portion.
Preferably, the outer circumference portion of the first optical lens and the second optical lens are bonded via an intermediate member so that the convex portion of the first optical lens faces the second optical lens.
Preferably, the second optical lens has a first convex portion on one surface, a second convex portion on the other surface opposing the one surface, and an outer circumference portion positioned around the first and second convex portions, and center axes of the first and second convex portions coinciding or substantially coinciding; and the outer circumference portion of the second optical lens and the outer circumference portion of the first optical lens are fixed in place.
According to a second aspect of the present invention, there is provided a method of producing an optical system having first and second optical lenses, wherein the first optical lens has a substrate comprised of an optical material; and the substrate has a convex portion serving as a convex lens and an outer circumference portion positioned around the convex portion, a thickness of the substrate at the outer circumference portion being greater than a thickness of the substrate at the convex portion; comprising the step of bonding together the outer circumference portion of the first optical lens and the second optical lens so that optical axes of the first and second optical lenses coincide or substantially coincide.
Preferably, the method further includes the step of mounting the outer circumference portion of the second optical lens on the outer circumference portion of the first optical lens and positioning the first and second optical lenses so that the optical axes coincide or substantially coincide; a mounting surface of the outer circumference portion of the first optical lens being flat or approximately flat; and a bottom surface of the outer circumference portion of the second optical lens being flat or approximately flat.
More preferably, the mounting surface of the outer circumference portion of the first optical lens is vertical or substantially vertical with respect to the optical axis of the first optical lens; and the bottom surface of the outer circumference portion of the second optical lens is vertical or substantially vertical with respect to the optical axis of the second optical lens.
Preferably, the first optical lens further comprises a flat portion positioned around the convex portion; the outer circumference portion of the first optical lens is positioned around the flat portion; and a thickness of the substrate at the outer circumference portion of the first optical lens is greater than a thickness of the substrate at the flat portion.
For example, the outer circumference portion of the first optical lens and the second optical lens are bonded via an intermediate member so that the optical axes of the first and second optical lenses coincide or substantially coincide.
For example, the second optical lens has a first convex portion on one surface, a second convex portion on the other surface opposing the one surface, and the outer circumference portion positioned around the first and second convex portions and center axes of the first and second convex portions coinciding or substantially coinciding.
According to a third aspect of the present invention, there is provided an optical pickup comprising a laser; an optical system for focusing laser light from the laser on an optical disk; and a photodetector for receiving the laser light reflected at the optical disk; wherein the optical system comprises first and second optical lenses arranged so that their optical axes coincide or substantially coincide; the second optical lens passes the laser light from the laser and supplies it to the first optical lens; the first optical lens has a substrate comprised of an optical material; the substrate has a convex portion for focusing laser light from second optical lens on the optical disk and an outer circumference portion positioned around the convex portion; a thickness of the substrate at the outer circumference portion is greater than a thickness of the substrate at the convex portion; and the outer circumference portion of the first optical lens and the second optical lens are fixed in place so that the convex portion of the first optical lens faces the second optical lens.
Preferably, the outer circumference portion of the first optical lens faces the outer circumference portion of the second optical lens; and a facing surface of the outer circumference portion of the first optical lens and a facing surface of the outer circumference portion of the second optical lens are flat or approximately flat.
More preferably, a facing surface of the outer circumference portion of the first optical lens is vertical or substantially vertical with respect to the optical axis of the first optical lens; a facing surface of the outer circumference portion of the second optical lens is vertical or substantially vertical with respect to the optical axis of the second optical lens; and the facing surface of the outer circumference portion of the first optical lens and the facing surface of the outer circumference portion of the first optical lens are bonded together.
Preferably, the first optical lens further comprises a flat portion positioned around the convex portion; the outer circumference portion is positioned around the flat portion; and a thickness of the substrate at the outer circumference portion is greater than a thickness of the substrate at the flat portion.
For example, the outer circumference portion of the first optical lens and the second optical lens are bonded via an intermediate member so that the convex portion of the first optical lens faces the second optical lens.
For example, the second optical lens has a first convex portion on one surface, a second convex portion on the other surface opposing the one surface and an outer circumference portion positioned around the first and second convex portions, and center axes of the first and second convex portions coinciding or substantially coinciding, and the outer circumference portion of the second optical lens and an outer circumference portion of the first optical lens are bonded together.
In short, the substrate of the first optical lens has a convex portion serving as a convex lens and an outer circumference portion arranged around the convex portion. The thickness of the outer circumference portion of the substrate is greater than that of the convex portion of the substrate.
By fixing the outer circumference portion of the first optical lens and the second optical lens so that the convex portion of the first optical lens faces the second optical lens, the trouble in assembling the optical system can be reduced comparing with the case where a convex lens is bonded to the lens holder and the lens holder is fixed to the second optical lens. Also, an optical system having a high NA can be obtained.